In high voltage direct current (HVDC) power transmission systems, two basic power converter technologies are used to transform the voltage from AC to DC and vice-versa. One converter technology or type is the line-commutated current source converter (CSC) and the other technology is the force-commutated or self-commutated voltage source converter (VSC). An overview of today's HVDC systems including the two technologies as well as their applications is for example given in “The ABC of HVDC transmission technologies”, IEEE Power & Energy Magazine March/April 2007, vol. 5, no. 2.
A simplified single-line diagram of a commonly known power conversion system with CSC, which is used in HVDC systems, is shown in FIG. 1. The CSC 1 comprises two three-phase, six-pulse bridges 2, each of the bridges 2 comprising six thyristor valves. The DC terminals of the two bridges 2 are connected in series and their DC output is coupled on one side to a DC transmission line or cable 3 and on the other side to ground 4. A DC filter 5 smoothes harmonics in the DC voltage. The AC terminals of the two bridges 2 are each connected via a transformer 6 to an AC bus 7. A shunt-connected AC filter 8 filters AC voltage harmonics.
In WO 96/09678, a power conversion system with CSC is disclosed, as shown in FIG. 2, which additionally to the system of FIG. 1 includes a series capacitor 9 in each of the phases, where the capacitors 9 are placed between the AC terminals of the two converter bridges 2 and the transformer. Instead of two two-winding transformers 6 a three-winding transformer 10 is used. The series capacitors 9 provide reactive power in order to compensate for the reactive power generated by the CSC 1.
Such reactive power compensation is not needed when a VSC instead of a CSC is used, since the VSC allows the independent control of active and reactive power so that the reactive power can be taken care of by the VSC control directly.
A simplified single-line diagram for a three-phase power conversion system with VSC, which is used in modern HVDC systems, is depicted in FIG. 3. The VSC 11 comprises converter valves 12 connected in a known six-pulse-bridge configuration, where the converter valves 12 each comprise an IGBT 13 (Insulated Gate Bipolar Transistor) in anti-parallel connection with a free-wheeling diode 14. The VSC 11 is connected on its AC side to a converter reactor 15, followed by a shunt-connected harmonic filter 16 and a transformer 17. The converter reactor 15 blocks harmonic currents arising from the switching of the VSC 11, and together with the harmonic filter 16 it protects transformer 17 from any high frequency components. The transformer 17 is coupled to an AC bus 18. Two identical, series-connected capacitor units 19 are connected between a first pole 20 and a second pole 21 on the DC side of the VSC 11 in order to provide a stiff DC voltage source. The midpoint between the two capacitor units 19 may be connected to ground. The DC voltage between the two poles 20 and 21 is symmetrically balanced between a positive voltage level +UDC and a negative level −UDC. Accordingly, the DC potential at connection point 28 is zero. Since connection point 28 represents the AC phase terminal of VSC 11, it can be noted that no DC potential occurs on the AC side of VSC 11, i.e. the converter reactor 15, the harmonic filter 16 and the transformer 17 all “see” only a pure AC voltage.
The power conversion system according to FIG. 3 represents a very cost effective and good solution for comparatively lower power. If a higher power is required, it is known to use two or more series-connected converter bridges 22, as is shown in FIG. 4. This configuration results in a reduction of the power, and thereby of the switching voltage, per bridge 22, which reduces the switching losses of the VSC 23. Additionally, the switching frequency can be lowered since it becomes possible to switch between the bridges 22, as is for example described in DE 101 03 031 A1. The reduced switching frequency leads to the cancellation of lower order harmonics. In FIG. 4, the same arrangement of converter reactor 24, harmonic filter 25 and transformer 26 is used on the AC side of each of the bridges 22 as was used in case of the single bridge of FIG. 3. The AC side of bridges 22 is connected in parallel to AC bus 27. The converter reactors 24 and harmonic filters 25 are again needed in order to not expose the transformers 26 to the switching frequency. The disadvantage which such a configuration is that in case of n series-connected bridges also n converter reactors and n harmonic filters are needed which increases the costs considerably. Additionally it is to be noted that a DC potential occurs at the AC phase terminals 29 of bridges 22, since zero DC potential is now developed at the midpoint 30 between the two bridges 22. The midpoint 30 can also directly be connected to ground. This results in the necessity to design the transformers 26 for DC, i.e. no standard transformers can be used, which constitutes a further cost factor.